Abstract 340: γ-carboxylated Growth Arrest Specific Protein 6 Differentially Induces Calcification In Human Aortic Valves

Introduction: Growth arrest specific protein 6 (GAS6) is a secreted γ-carboxylated vitamin K-dependent protein with roles in cell proliferation and migration. The role of GAS6 and post-translational γ-carboxylation in calcific aortic valve disease (CAVD) has yet to be evaluated. Hypothesis: Using proteomics, we identified GAS6 as a novel protein abundant in valve tissue and valvular interstitial cells (VICs), and assessed the hypothesis that γ-carboxylated GAS6 has a mechanistic role in the development of CAVD. Methods: Whole aortic valve and tissue extracellular vesicle (EV) label-free proteomics, valve histopathology, and valvular interstitial cell isolation was performed on human specimens collected from surgical aortic valve replacements for aortic stenosis at Brigham and Women’s Hospital. EVs were isolated from whole valves using ultracentrifugation and Optiprep density gradients. Mass-spectrometry will confirm γ-carboxylation. Results: GAS6 abundance was elevated in valvular areas of calcification compared to non-diseased regions (log2 FC=5, q=4.2E-10), and were enriched in GO terms associated with exosome and vesicular transport (N=18). Due to the role of EVs in ectopic cardiovascular calcification, tissue EVs were isolated from calcified aortic valves (N=9), and proteomics identified GAS6 in the vesicle-enriched fractions. Immunohistopathology of GAS6 in CAVD specimens identified co-localization of GAS6 with inflammation and calcification. Protein expression of GAS6 from VICs was significantly increased by media supplemented with organic phosphate (β-glycerophosphate) (N=5-8, p<0.05), but markedly decreased by supplementation with inorganic phosphate (NaH 2 PO 4 ) (N=5-8, p<0.05), despite both conditions generating Alizarin red positive calcification. GAS6 knockdown prevented calcification only in the setting of organic phosphate stimulation. Conclusions: GAS6 is a novel protein of pathological significance in CAVD. EV enrichment of GAS6 and unappreciated differential mechanistic roles of phosphate sources impacts valvular calcification, potentially through γ-carboxylation regulation. GAS6 has an unrecognized role in CAVD, which may have important implications for future disease pharmacotherapy.